Abstract P3-05-22: PI3K pathway as a novel target in androgen-driven aromatase inhibitor resistant breast cancer

Abstract

Abstract Breast cancer has the highest incidence and second highest mortality rate among all cancers in women worldwide (1). The gold standard treatment for post-menopausal breast cancer is aromatase inhibitor (AI) therapy. Unfortunately, resistance is inevitable in about 30-60% of these patients (2). Research from our lab has previously identified a transcription factor, HOXC11, to be associated with breast cancer resistance and metastases (3). Prosaposin (PSAP) was then identified as a putative HOXC11 target gene in the AI resistant setting. PSAP is a secreted protein that also plays a role in metastatic prostate cancer in both a ligand dependent and independent fashion (4,5). We have previously shown PSAP to be an androgen responsive gene that can upregulate androgen receptor (AR) expression in a ligand dependent manner in AI resistance. An AR antagonist was effective at slowing down cell proliferation in AI resistant cells in vitro . However, since PSAP can also potentially act in a ligand independent fashion, this current study is focused on evaluating the role of PSAP in PI3K pathway activation in the development of AI resistance. Treatment of cells with recombinant PSAP protein (rhPSAP) resulted in increasing expression of p-AKT in a dose-dependent manner. To target this pathway of resistance, we used a pan-class PI3K/mTOR inhibitor BEZ235 on our AI resistant cells. Functional studies with BEZ235 treatment showed significant reduction in both cell motility as well as cell proliferation. Treatment of AI resistant cells with BEZ235 resulted in decreased expression of p-AKT, with little or no effect on AR expression. These findings suggest that to prevent resistance to AI therapy, combination treatment regimens including AR antagonists plus a PI3K inhibitor may ensure sustained response to therapy.